Assume versions are a pre-release if we cannot parse them

* The most likely unparseable version is something like "dev" or
  "py6-dev" etc. The number of sane version numbers that are
  not normalizable and should be conisdered stable are fairly low.
Merge pull request #908 from pypa/dstufft/default-prerelease-true

Assume versions are a pre-release if we cannot parse them